The present disclosure relates to an incandescent lamp, more specifically, to a filament coil structure with a different geometry. The filament coil structure finds particular application in vehicle headlamps or headlights, although it will be appreciated that selected aspects may find application in related applications encountering issues of increasing contrast of the cut-off region.
U.S. Publication No. 2006/0038471 and U.S. Pat. No. 7,456,558 disclose incandescent lamp filaments having different pitch ratios located at opposite ends of the filament in order to increase the luminance at the light/dark boundary of the low beam. The pitch ratio is represented by the following equation:PR=h/d 
wherein:
PR=Pitch Ratio
d=Diameter of filament wire
h=Distance between adjacent coil turn axis
However, at the ends of the incandescent filament close to the base, the pitch ratio is higher resulting in less light onto roadways. Other incandescent filament technologies have outside diameters increase continuously from a minimum value at a first end of the filament to a maximum value at the second filament end resulting in a less compact vehicle headlamp.
Even in light of recent advances, the industry continues to lack a filament with useful geometry for achieving high luminance in the region of the light/dark boundary of the low beam vehicle headlight where good illumination is desired.